ERBB2 and maternal uniparental disomy of chromosome 20: In the past, patients with HER2-positive breast cancer generally had unfavorable outcome compared with HER2 negative cancers, but the prognosis of HER2-positive locally advanced or metastatic breast cancers (MBCs) has dramatically improved due to the introduction of trastuzumab, pertuzumab, and trastuzumab emtansine (T-DM1).